In the field of mechanical devices used, for example, in aircraft and automobiles, many actuation oils are used as the pressure transmission oils. Of these actuation oils, the actuation oils for use in aircraft and automobiles must have a low viscosity and small temperature dependency, as well as an excellent lubrication characteristic and excellent oxidation stability over a broad temperature range of from a low temperature to a high temperature.
In the prior art, as the actuation oil in the above fields, an extremely low viscosity silicone oil or low viscosity mineral oil is utilized, and primarily glycol ethers are used as brake fluids. However, although an extremely low viscosity silicone oil may have an excellent viscosity characteristic, the compression ratio thereof is high and the lubrication characteristic is unsatisfactory. Also, a low viscosity mineral oil does not have a satisfactory low temperature viscosity characteristic or oxidation stability, and glycol ethers not only have an unsatisfactory low temperature viscosity, but also have a lower boiling temperature caused by moisture absorption stemming from a high hygroscopic property. Accordingly, there is a strong demand for an actuation oil composition having an excellent viscosity characteristic, lubrication characteristic, and oxidation stability, and no denaturation caused by moisture absorption.
In the prior art, several proposals have been made concerning a synthetic lubricant oil comprising a liquid low molecular weight polymer. For example, Japanese Examined Patent Application (Kokoku) No. 47-21650 proposes the use of a copolymer distillate of an ethylene-propylene copolymer containing ethylene components at a specific ratio obtained by polymerization from which the low boiling distillate has been removed by non-destructive rectification or rectification after applying treatments such as cracking, hydrogenation, and dewaxing extraction. However, such a synthetic lubricant oil comprising a copolymer distillate is not a lubricant having an excellent balance of properties such as viscosity index, pour point, ignition point, shear stability, oxidation stability, thermal stability, oil film strength and further, even if the copolymer distillate can be utilized as an actuation oil, it cannot be utilized as an actuation oil having an excellent viscosity characteristic, lubrication characteristic, and oxidation stability over a broad temperature range.
Also, the present Applicant has proposed, in Japanese Unexamined Patent Application (Kokai) No. 57-117595, an ethylene-.alpha.-olefin copolymer synthetic lubricant oil containing 30 to 70 mol% of ethylene having a number average molecular weight of 300 to 2000, and a statistic molecular weight distribution with a Q-value (i.e., weight average molecular weight/number average molecular weight) of 3 or less. The ethylene-.alpha.-olefin copolymer according to this proposal has an excellent viscosity index, pour point, ignition point, shear stability, thermal stability, and oil film strength, and can exhibit an excellent performance when used as a synthetic lubricant. However, when this copolymer is utilized as an actuation oil, it is still unsatisfactory from the viewpoint of the viscosity characteristic over a broad temperature range.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 48-65205, Japanese Unexamined Patent Publication (Kokai) No. 51-112809, and Japanese Examined Patent Publication (Kokoku) No. 47-42723 all propose lubricant oil compositions in which specific low crystalline ethylene-propylene copolymers are formulated as an viscosity index improver in lubricant oils. However, even if these lubricanting oils are utilized as actuation oils, they do not have a satisfactory viscosity characteristic, lubrication characteristic and oxidation stability over a broad temperature range.